1. Field of the Invention
This invention relates to a liquid crystal display panel, and more particularly to an arrangement of thin-film transistors suitable for being used in a liquid crystal display device for displaying a picture free from non-uniformity in a manufacturing process.
2. Description of the Related Arts
In the liquid crystal display (referred to LCD) panel, a thin-film transistor array is used for switching the liquid crystal in each pixel element. The thin-film transistors in the array have been increased in number and miniaturized, as the displaying area in the LCD panel has been widened. The high-integration and the miniaturization require severe preciseness in manufacturing process. A small deviation in the manufacturing process affects the quality of an imaged picture. Especially, in a color LCD panel, the degradation of the imaged picture quality is remarkable.
More specifically, a plurality of drain buses and a plurality of gate buses are orthogonally crossed on a glass substrate to form a mesh. Each pixel electrodes is formed on an area enclosed in the mesh. The pixel elctrodes operate as source electrodes of the thin-film transistors. The gate buses have branches extending along a predetermined side portion of the pixel electrodes. The drain buses have protrusions facing the predetermined side portion of the pixel electrode through the branches of gate buses. The branches of gate buses are separated from the pixel electrodes and the protrusions of drain buses through amorphous silicon film, thereby forming thin-film transistors with a drain electrode of the protrusion, a gate of the branch and the source of the predetermined side portion.
In such thin-film transistor array of the prior art, however, all the transistors are formed uniformly. The position and direction of the drain, the gate and the source are the same in all pixel electrodes. Therefore, a deviation of etching mask alignment varies stray capacitances between gate and drain electrodes and between gate and source electrodes. This variation in stray capacitances causes a change in brightness between all LCD panels. Moreover, since the LCD pixels are driven by a voltage having alternate polarities, the drain and source electrodes change their function with the alternation of the driving voltage polarity. This means that the brightness of the imaged picture changes with the alternation of the voltage polarity resulting in a flicker of a picture. In a color LCD device, this flicker causes a periodical color difference which, in turn, causes a remarkable degradation of color picture quality. Furthermore, in the LCD device for TV picture, each pixel electrode are driven by two thin-film transistors formed on opposite sides of the pixel electrodes and driven by two successive scanning periods. The deviation of the etching mask causes a difference of in the stray capacitances of the two thin-film transistors and results in a change in brightness in the two scanning periods. This brightness change also degrates the color picture quality.
In order to prevent the variation of the stray capacitances, U.S. Pat. No. 4,597,001 proposed to widen the gate electrode so that the positions of the drain and source electrodes may move on the gate electrode. This measurement makes each thin-film transistor large and associates with enlarged stray capacitances.